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Activated carbon supported Ni-Mo: Effects of pretreatment and composition on catalyst reducibility and on ethylene conversion

Abstract:

Activated carbon (AC) supported Mo, Ni and Ni-Mo catalysts have been studied by means of TPR and the catalytic transformation of ethylene as a probe reaction. The initial state of the catalysts varied with different reducing or sulfiding pre-treatments. TPR of the fresh samples suggested that Ni and Mo are well dispersed on the support. Pre-reduction at 500°C produced Mo surface sites with average oxidation number close to two and Ni metallic species. For the sulfided catalysts low peak temperatures were observed, which seem to relate with easily removable sulfur. Activity results correlated well with the TPR spectra. Contrary to what is normally found in Al2O3 or SiO2, both the TPR and activity/selectivity characteristics of Ni seem to dominate over those of Mo in unsulfided samples, which is likely to be due to the lower interaction of Ni with the carbon support. Coke was an important product for most of the samples, but the Ni-Mo combination in general lowered its formation. However, previous reports suggesting that carbon-supported HDS catalysts are less prone to coke deposition than the Al2O3 supported ones could not be substantiated. Regardless of the type of pre-treatment, AC and Mo/C showed low activity with 100% selectivity to coke, except the sulfided Mo/C samples, where ethane production was observed. Hydrogenation sites of sulfided Mo/C were very fragile, as mild post-reduction produced complete deactivation at steady state conditions. However, in sulfided but not post-reduced samples, the feed can protect active sites from the deleterious effect of the H2 atmosphere. In the case of unsulfided samples hydrogenation and cracking were associated to Ni, but sulfiding inhibited these functions of Ni sites in Ni/C. High selectivity to ethane was observed in sulfided NiMo/C, showing that this catalyst is more resistant to sulfur than the single metal ones. From the dependence of selectivity of sulfided catalysts on the type of pre-treatment it is concluded that the hydrogenation sites in NiMo/C are of the same type as those of Mo/C, i.e., they are associated mainly to Mo sulfides.